Automatic merge from submit-queue (batch tested with PRs 38608, 38299)

controller: set unavailableReplicas correctly when scaling down

```
deployment_controller.go:299] Error syncing deployment
e2e-tests-kubectl-2l7xx/e2e-test-nginx-deployment:
Deployment.extensions "e2e-test-nginx-deployment" is invalid:
status.unavailableReplicas: Invalid value: -1:
must be greater than or equal to 0
```

The validation error above occurs usually when a Deployment is
scaled down. In such a case we should default unavailableReplicas
to 0 instead of making an invalid api call.

@kubernetes/deployment